The covering of substrates such as conductors or cores for use in communications with plastic insulating or jacketing materials is generally accomplished with pressure or tubing extrusion tooling. In pressure extrusion, a substrate is moved through a core tube having an opening that is only slightly larger than the substrate. The end of the core tube is positioned within a die cavity and spaced from a land of a die through which the substrate and the plastic extrudate are moved. Pressure extrusion results in a well defined insulative cover which is disposed tightly about the substrate.
In normal pressure extrusion tooling, the moving substrate is exposed to a relatively high melt pressure of the plastic material in a so called "gum space" between the end of the core tube and the die land. Flow of the plastic material is comprised of two components--differential pressure flow and drag flow. The pressure flow is caused by the difference in pressure between the entrance to the land and the exit orifice of the die. Drag flow is defined as the volumetric forward displacement of a viscous material between a stationary and a moving surface such as between the land and the substrate. See. E. I. Bernhardt Processing of Thermoplastic Materials which was published in 1974 by Krieger Publishing Company.
This relatively high melt pressure requires that the inside surface of the core tube be only slightly larger than the outer dimension of the substrate. This avoids any problems in concentricity of the insulation cover and creates a seal which prevents the extrudate from flowing in a direction opposite to the direction of advance of the substrate and into the core tube. Typically, the clearance between the substrate's outer surface and the inner surface of the core tube is 0.001 to 0.002 inch for product sizes in the 0.015 to 0.075 inch range.
Unfortunately, this relatively small clearance prevents any substrate irregularities such as intermittent oversize sections from passing through the tooling. Consequently, particular substrates having a non-uniform cross-section or any spliced, relatively smooth cores cannot be insulated using conventional pressure extrusion techniques. If the core tube is not oversized to accommodate these irregularities, the substrate will break, requiring down time for operator string-up. On the other hand, if the core tube is oversized, the pressure in a conventional pressure extrusion process will cause a backflow of the plastic material into the core tube.
One such substrate having irregularities is that of a conductor of a telephone cord which is used with customer station equipment. A telephone cord conductor generally comprises a polymeric core having a plurality of tinsel ribbons wrapped helically thereabout. Telephone cords are well disclosed in the prior art such as, for example, U.S. Pat. No. 3,037,068 issued May 29, 1962 in the name of H. L. Wessel, and in U.S. Pat. Nos. 2,920,351 and 3,024,497 issued on Jan. 12, 1960 and Mar. 13, 1962 respectively in the names of E. C. Hardesty and D. L. Myers. Because a tinsel conductor is made with something less than a constant cross-section, the core tube must be oversized.
For these kinds of products, the art has resorted to tubing processes in which the leading end of the core tube generally is flush with or extends beyond the die opening. See U.S. Pat. No. 3,554,042 which issued on Jan. 5, 1971, in the name of E. R. Cocco. But in commonly assigned, copending application Ser. No. 229,434 which was filed on Feb. 29, 1981 now U.S. Pat. No. 4,339,298, the downstream end of the core tube is positioned within the die land. In a tubing operation, the clearance between the inner surface of the core tube and the outer dimension of the substrate, such as an array of tinsel conductors, is large enough to permit oversize substrate sections to be passed through the core tube without jamming. Unlike pressure extrusion, tubing relies solely on differential pressure flow and the extrudate is drawn down about the substrate externally of the die.
A tubing process does not always result in the most acceptable product since tubed covers generally have more size variations and irregular surfaces and are not disposed as tightly about the substrate as in a pressure extrusion process. It should be clear that irregular or intermittently oversized substrates which are necessarily tube-insulated or jacketed are done so at some expense to the overall product configuration and/or performance.
This disadvantage of a tubing process has been aggravated because of recent changes in the materials which are used for insulation and jacketing. These changes in materials, at least for cords, have come about because of a somewhat recently introduced cord connection arrangement, which is referred to as modularity. Miniature plugs are connected to each end of a cord to facilitate attachment to jacks in telephone instruments and in wall outlets. For example, see U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973 respectively in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr. and S. W. Walden and U.S. Pat. No. 4,148,359 issued Apr. 10, 1979 in the name of E. C. Hardesty. With the introduction of modularity, it became necessary to use a different cord construction because of a need for a smaller cross-section to be compatible with the plugs. In order to reduce the size of the insulated conductor, the tinsel is insulated with a crystalline, relatively high molecular weight plastic material as disclosed and claimed in U.S. Pat. No. 4,090,763 which was issued on May 23, 1978 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman and which is incorporated by reference hereinto. A material such as that disclosed in the above-identified Congdon et al application is available commercially from E. I. duPont Company under the trade name Hytrel.RTM. polyester elastomer.
Extrusion of the above-identified plastic material is characterized by rapid changes in melt viscosity and melt strength with slight variations of polymer temperature. For relatively high molecular weight and/or branched polymers such as Hytrel.RTM. polyester elastomer material, the melt viscosity increases significantly as the pressure increases. These characteristics could cause non-uniform wall thickness and polymer flow pulsations unless suitable control is exercised.
The prior art also shows techniques for controlling the engagement of a tubed Hytrel.RTM. plastic extrudate with the core being enclosed. In U.S. Pat. No. 4,206,611, which issued on June 3, 1980 in the names of W. M. Kanotz et al, an extruded tubular covering is held out of contact with an advancing conductor until the extrudate becomes sufficiently form-sustaining by suitable crystallization. Then, when the crystallized insulation is drawn down on the conductor, any tinsel burrs which protrude outwardly are compressed. This results in a conductor having a continuously concentric insulation and a uniform wall thickness.
There are insulating operations other than these which are used to cover tinsel conductors in which problems have developed because of the plastic which is extruded. For example, a low resistance cord may include a plurality of conductors each comprising wires which are stranded together and insulated. Relatively high pressures are required to extrude some plastic materials such as the hereinbefore-mentioned Hytrel.RTM. plastic material. Such plastic materials have a high molecular weight and are polymerically branched, and normal pressure extrusion techniques may cause dramatic melt viscosity and shear stress increases thereby causing melt fracture.
Melt fracture of particular plastic materials during extrusion is a structural breakdown by fracture within a polymer melt where the critical shear stress becomes abnormally independent of extrusion die orifice size. The result is an insulation cover which is extremely irregular and totally unacceptable.
And yet, these plastic materials such as Hytrel.RTM. elastomers have much to offer. They generally are tough and mechanically resistant to many of the conditions encountered by insulated substrates in the field. It is highly desirable to be able to take advantage of these benefits; but to do so, the problem of melt fracture must be overcome.
It should be clear, that there are several problems in the extrusion of particular plastic materials which must be addressed. Moreover, extrusion techniques need to be reexamined to find solutions to problems caused during the covering of non-uniform substrates. Prior art extrusion technology seemingly lacks tooling which is capable of extruding a substantially uniform, substantially concentric wall about a substrate which is irregular or which includes intermittent oversized portions.